Adaptive cruise control (ACC) where the vehicle speed of the host vehicle is controlled in accordance with the vehicle speed of a vehicle ahead using a radar, etc., has seen application in products. Further, in recent years, there has been proposed an ACC technique in which a curve ahead of the host vehicle is detected using a navigation system, and deceleration is automatically performed at the curve.
With such systems where the vehicle is controlled or a warning is given to the driver based not only on the travel state of the host vehicle but also on information on road shapes, etc., there are problems such as the occurrence of a lag in control/warning timing, etc., due to such influences as detection errors with respect to road shapes, changes in road shapes due to construction work, calculation errors with respect to the distance from the host vehicle to a predetermined site, e.g., a curve, etc., and so forth. For this reason, a technique for measuring road shapes in real time and with high precision becomes important.
As a technique developed for such purposes, the technique disclosed in Patent Document 1 has conventionally been known. The conventional technique disclosed in Patent Document 1 is one in which stationary objects ahead of a vehicle are detected using a millimeter-wave radar, and, from among the detected stationary objects, only those stationary objects that are effective for road shape estimation are selected using previous processing results, and the yaw rate sensor and steering angle sensor of the vehicle, thereby estimating road shapes, such as curves, etc. In addition, it is one in which, instead of using previous processing results, a plurality of road shapes are hypothesized, effective stationary objects are selected per road shape, and those that resulted in the greatest number of selected stationary objects are selected as stationary objects that are effective for road shape estimation.    Patent Document 1: JP Patent Application Publication (Kokai) No. 2007-66047 A